1. Field of the Invention
The present invention relates to a semiconductor device of a band-to-band tunneling type.
2. Description of the Related Art
There has been proposed a new type transistor utilizing the band-to-band tunneling (BBT) phenomenon. In this BBT transistor, there does not occur a problem of the well known short channel effect in the widely used MOS FET, and thus it can be minuaturized to a larger extent than the MOS FET.
FIG. 1 is a cross sectional view showing an example of a known BBT transistor. The transistor comprises a P.sup.- silicon substrate 1, a P.sup.+ drain region 2 and an N.sup.+ source region 3, these regions being formed in one surface of the silicon substrate. The drain region 2 and source region 3 are separated from each other such that a channel region 4 is formed therebetween. On the silicon substrate 1 there is formed a silicon oxide film 5 and on this silicon oxide film there is provided a gate electrode 6 such that the gate electrode covers a part of the drain region 2 and the channel region 4. In this known BBT transistor, by applying a suitable reverse potential between the drain region 2 and the source region 3 and by applying a suitable bias potential to the gate electrode 5, there is produced a band bend in a surface region of a part of the drain region 2 underneath the gate electrode.
FIG. 2 is a schematic view illustrating the energy diagram of the above mentioned band bend produced in the interface region in the drain region 2 between the silicon substrate 1 and the silicon oxide layer 5 underneath the gate electrode 6. At the band bend, minority carriers, i.e. electrons are moved from the valence band into the conduction band by the tunneling phenomenon and are collected into the source region 3 via the channel region 4 to cause a current flowing from the source region 3 to the drain region 2.
In the above mentioned known BBT transistor, the band bend is realized in the depletion region formed in the surface region of the silicon substrate 1 and the height V.sub.bend of the band bend is limited by the impurity concentration in the drain region 2, so that the band bend could not be made large under the given bias condition. That is to say, when the impurity concentration is low, the depletion region is expanded and the strength of the electric field which relates to the inclination of the band bend is reduced, so that the inclination of the band bend is decreased although the height V.sub.bend of the band bend can be made large as shown in FIG. 3A. Therefore, the tunneling is hardly generated. When the impurity concentration of the drain region 2 is increased, the electric field becomes strong so that the inclination of the band bend is increased, but the height V.sub.bend of the band bend is decreased as illustrated in FIG. 3C, because the depletion region becomes small due to increased depletion layer capacitance. When the height V bend of the band bend becomes smaller than the band gap, the tunneling does no more occur. In this manner, in order to perform the tunneling sufficiently, both the inclination and the height of the band bend have to be increased. However, in the known BBT transistor these conditions could not be satisfied simultaneously. Therefore, in the known BBT transistor, a significant tunneling current can be obtained only in a very critical condition depicted in FIG. 3B, said critical conditions corresponding to a very narrow range of the impurity concentration such as 2.about.5.times.10.sup.18 atoms/cm.sup.3 and more particularly about 3.times.10.sup.18 atoms/cm.sup.3. Even in this impurity concentration range, the tunneling current is very small such as several nano amperes. Therefore, a sufficiently large current driving property could not be obtained and a high speed switching operation could not be realized.